Effect of differential uniform temperature with thickness-wise linear temperature gradient oninterfacial stresses of a bi-material assembly

The thermal mismatch induced interfacial stresses are one of the major reliability issues in electronic packaging and composite materials. Consequently an understanding of the nature of the interfacial stresses under different temperature conditions is essential in order to eliminate or reduce the risk of structural and functional failure. Approach: In this analysis, a model was proposed for the shearing and peeling stresses occurring at the interface of two bonded dissimilar materials with the effect of different uniform temperatures in the layers. The model was then upgraded by accounting thickness wise linear temperature gradients in the layers using two temperature drop ratios. The upgraded models were then compared with the existing uniform temperature model. The proposed model can be seen as a more generalized form to predict interfacial stresses at different temperature conditions that may occur in the layers. Results: The results were presented for an electronic bi-material package consisting of die and die-attach. Conclusion: The numerical simulation is in a good matching agreement with analytical results

Modelling residual stresses and environmental degradation in adhesively bonded joints

The aim of this research was to develop predictive models for residual stresses and environmental degradation in adhesively bonded joints exposed to hot/wet environments. Different single lap joint configurations and a hybrid double lap joint with dissimilar adherends (CFRP/AIIFM73 double lap joint), were exposed to different ageing environments in order to determine the durability of the joints and the effects of ageing on the failure load. Thermal residual stresses in bonded joints were investigated with analytical solutions and finite element modelling, first with a bimaterial curved beam to validate the modelling process and determine the most suitable method for calculating thermal stresses in bonded joints. It was found that none of the analytical solutions and 2D geometric approximations was fully able to describe the 3D stress state in the strip. The incorporation of geometric and material non-linearity into the models was necessary to obtain accurate results. The validated methods were then used predict the thermal residual stresses in bonded lap joints. The thermal stresses were found to be highest in joints with dissimilar adherends. Moisture uptake in bonded joints was investigated using Fickian diffusion modelling. Gravimetric experiments were used to determine the Fickian diffusion parameters for the bulk adhesive and composite adherends. Transient diffusion modelling was used to predict the uptake in bonded joints. It was seen that moisture diffusion is a fully three dimensional process, and the effects of moisture absorption can only be adequately studied using 3D FEA. The effects of swelling from moisture absorption in bonded joints were investigated using coupled stress-diffusion FEA models. Coupled stress-diffusion 3D FEA was used to predict the transient and residual hygroscopic stresses that develop in bonded lap joints as a function of exposure time in accelerated ageing environments, taking into account the effects of moisture on the expansion and mechanical properties of the adhesive and CFRP substrate. It was seen that moisture absorption induces significant stresses in the joints and markedly different behaviour was seen in the cases of absorbent and non-absorbent adherends. Hygro-thermo-mechanical stresses arising from the exposure of single and double lap joints with thermal residual stresses to hot/wet environments were investigated. In the single lap joints, a reduction in the stresses present in the adhesive was predicted, owing to swelling of the adhesive from moisture absorption. In the double lap joint with dissimilar adherends, exposure to hot/wet environments initially reduced the stresses in the joint when dry, followed by an increase in the magnitude of some stress components and reductions in others with increasing levels of moisture absorption. This led to a higher equivalent stress state in the adhesive than when dry. Thermal residual and mechanical strains predictions were validated with internal strains measured by neutron diffraction and surface strains measured by moire interferometry. Comparisons of predicted and measured thermal residual strains showed low levels of strain in joints with similar adherends. The magnitude of strains in the CFRP/AI double lap joint was significant, with the same spatial distribution and magnitude in both measured and predicted strains. The comparison of mechanical strains predicted by FEA and measured strains by moire interferometry showed good agreement. High magnification moire interferometry also confirmed the location of strain concentrations predicted by FEA. A path independent cohesive zone model (CZM) and a coupled continuum damage model were used to predict and characterise damage and failure initiation in bonded joints. Progressive failure prediction was calibrated in the cohesive zone model using the moisture dependent cohesive fracture energy of FM73. There was a reasonably good agreement with the experimental failure loads. This implementation of the cohesive zone model is limited by the ability of the interface elements used, thereby creating mesh dependency. The Gurson-Tvergaard-Needleman (GTN) coupled damage model was used to predict the effects of residual stresses on failure loads. However, this method is difficult to implement, given the numerous parameters required. The failure loads predicted by the GTN model were comparable with the experimental data when the joints were dry or wet. The damage models were capable of predicting the sudden crack growth and propagation seen experimentally

Control of Mechanical and Fracture Properties in Two-phase Materials Reinforced by Continuous, Irregular Networks

Composites with high strength and high fracture resistance are desirable for structural and protective applications. Most composites, however, suffer from poor damage tolerance and are prone to unpredictable fractures. Understanding the behavior of materials with an irregular reinforcement phase offers fundamental guidelines for tailoring their performance. Here, we study the fracture nucleation and propagation in two phase composites, as a function of the topology of their irregular microstructures. We use a stochastic algorithm to design the polymeric reinforcing network, achieving independent control of topology and geometry of the microstructure. By tuning the local connectivity of isodense tiles and their assembly into larger structures, we tailor the mechanical and fracture properties of the architected composites, at the local and global scale. Finally, combining different reinforcing networks into a spatially determined meso-scale assembly, we demonstrate how the spatial propagation of fractures in architected composite materials can be designed and controlled a priori.Comment: 31 Pages, 4 Figures, 12 SI Figure

Delamination in reinforced concrete retrofitted with fiber reinforced plastics

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, 2000.Includes bibliographical references (leaves 251-269).The addition of fiber-reinforced plastic (FRP) laminates bonded to the tension face of concrete members is becoming an attractive solution to the rehabilitation and retrofit of damaged structural systems. Flexural strength is enhanced with this method but the failure behavior of the system can become more brittle, often involving delamination of the composite. This study investigates failure modes including delamination with the use of fiber reinforced plastics to rehabilitate various concrete structures. The focus is on delamination and its causes, specifically in the presence of existing cracks in the retrofitted concrete system. First, delamination processes in FRP retrofitted concrete systems are studied through combined experimental and analytical procedures. The delamination process is observed to initiate in the concrete substrate with micro cracks that coalesce into an unstable macro crack at failure. This macroscopic behavior is modeled through a finite element procedure with a smeared crack approach, which is found to be limited in the ability to represent the stress intensity at the delamination tip. For this reason it is shown that interfacial fracture mechanics can be used to describe the bimaterial elasticity and complex stress intensity at the delamination tip and provide a criterion governing the propagation of delamination using energy methods. Then, peeling processes occurring at existing cracks in the retrofitted system are studied through fracture mechanics based experimental and analytical procedures. An experimental program involving specialized shear notch specimens demonstrates that the location of the notch and laminate development length are influential on the shear crack peeling process. A finite element procedure is used to evaluate the crack driving forces applied at the shear notch crack mouth, and the fracture analysis is extended to evaluate initiation of peeling at the shear notch scenario. Finally, delamination failures in FRP retrofitted reinforced concrete beams representing "real-life" retrofit scenarios are investigated. An experimental and analytical program is conducted to investigate influences on the failure processes. The application of the fracture based peeling analysis to a quantitative design procedure is investigated, and a computational design aid to assist the iterative design procedure is developed.by Brian Phillip Hearing.Ph.D

Dynamic behaviour of composite sandwich beams and plates with debonds

Fibre Reinforced Polymer (FRP) composites are continuing to gain prominence in structural as well as non-structural applications all over the world due to their outstanding properties such as high strength to weight ratio, corrosion resistance, good thermal performance, anti-fire performance and reduction of carbon dioxide emissions both through its method of production and their effective thermal insulation qualities. The increased popularity and demand for FRP composites have spurred research efforts in both academia and civil construction industry. A composite sandwich structural element can be made-up by attaching two thin and stiff skins to a lightweight and thick core, which serves as a building block for constructing laminated structural sandwich composites for civil engineering applications. A structural composite multilayer beam or plate can be manufactured by gluing two or more composite sandwiches together to form a laminated composite. An Australian manufacturer has fabricated a new generation structural Glass Fibre Reinforced Polymer (GFRP) sandwich panel made from E-glass fibre skin and a high strength modified phenolic core for civil engineering applications, the outstanding features of the sandwich material being high strength to weight ratio, good thermal insulation and termite resistance. These features offer the composite panel a wide range of applications in Australian construction industry as structural elements such as beams, slabs, bridge decks and railway sleepers. While sandwich composite construction has some great benefits, the behaviour of sandwich structures containing damage is much more complex and one of the major factors limiting the optimum usage of the same. Although perfect bond between the skin and the core is a common assumption, an important issue that needs to be considered in using a composite beam or slab is the development of debonding between the skin and the core, which is a predominant damage mode of these sandwiches. Interlayer debonding or delamination is also a predominant form of damage phenomenon in laminated composites, which can often be pre-existing or can take place under service conditions. Debonding and delamination cause significant changes in the vibration parameters, such as natural frequencies and mode shapes of structures leading to serviceability issues related to deflection limits. During the design stages of FRP composite structures, it is vital to identify how the global response of these structures will be affected by skin-core debonding and interlayer delamination. Even though the dynamic behaviour of undamaged sandwich panels is the subject of extensive research, papers reported on the dynamic behaviour of sandwich panels with debonding are less presented in the literature. Specifically, knowledge on seismic behaviour of composites with debonds is severely limited. Further research is therefore needed into investigation of the dynamic behaviour of debonded composite structural elements to gain wider acceptance of composites by the structural composite field around the globe. Finite element method is particularly versatile and effective in the analysis of complex structural behaviour of the composite structures. The use of dynamic analysis methods helps the engineer to better understand the behaviour of a structure subjected to an earthquake. This research deals with the investigation of the influence of debonding on the dynamic characteristics of novel GFRP beams and plates by finite element based numerical simulations and analyses using STRAND7 finite element (FE) software package. The research approach is to develop a three dimensional computer model and conduct numerical simulations to assess the dynamic behaviour. The FE model developed has been validated with published experimental, analytical and numerical results for fully bonded as well as debonded beams and slabs. Dynamic seismic response investigation of structures containing GFRP slab panels with debonds subjected to a probable earthquake loading is also incorporated. The influence of various factors such as debonding size, location of debonding, boundary condition of the structural member and the effect of multiple debonding has been delineated with the aid of an extensive parametric investigation and comparative analyses. Generally it was evident from all the analyses that debonding and interlayer delamination cause reduction in magnitudes of natural frequency. Moreover, some vibration modes and accordingly the mode shapes are also noticeably changed. It is generally observed that higher natural frequencies and mode shapes are more influenced by the presence of debonding. Yet there are exceptions to this trend depending on how severely the local modes are affected by debonding. It is observed that the associated mode shapes explain the causes of these inconsistencies. Furthermore, the results show that the presence of relatively small debonding or delamination has an insignificant effect on the natural frequencies and associated mode shapes. The results also suggest that fastening the delamination region is an effective corrective measure in decreasing the natural frequency variation, hence improving its dynamic performance compared to the delaminated panel. To sum up, the results suggest that debonding and delamination predominantly leads to reduction of the natural frequencies and modifying the modes of vibrations thus altering the mode shapes as well, resulting in dramatic changes in dynamic characteristics when extents of debonding are large. The more the supports are restrained, the greater the influence on free vibration characteristics. Most importantly, the findings demonstrate the feasibility of non-destructive methods to detect debonding and delamination damage in practical composite structures. The results of the seismic study show that the seismic performance of the considered buildings is unresponsive to small percentages of debonding of the GFRP slab panels. An existence of extensive percentage of debonding causes a slight increase in the maximum vertical displacement and reduction of natural frequencies of the buildings due to loss of stiffness occurring due to debonding. The results of this study will offer engineers and designers a better understanding of the influence of debonding and delamination on the dynamic performance of FRP composites in general, in addition to its direct application to Australian composite industry. Finally, the study provides valuable insights into the seismic behaviour of composite slabs with debonding thus facilitating the actual application of these findings in worldwide composite industry

Computational Mechanics of Fracture and Fatigue in Composite Laminates by Means of XFEM and CZM

This thesis is on the computational fracture analysis of static and fatigue fracture in advanced composite materials using Extended Finite Element Method (XFEM). Both in analytical and numerical approaches, the techniques and procedures need adjustments to take account for numerous effects brought by the heterogeneous and orthotropic nature of the advanced composite materials. The frst part of this study is on the calculation of Energy Release Rate (ERR) for cracks in composite structures. J-Integrals are widely used in computational methods for the ERR evaluation however, they do not show consistency in structured materials when the crack is close to the material interfaces. Furthermore, when J-Integrals are implemented in XFEM, the enrichment functions of the crack-tip and the interfaces create even more complications. The outcome of the first study clarified that the linear elastic fracture mechanic (LEFM) approach on its own suffers from the effects caused by the crack-tip singularity and the stress field definition at the crack-tip. Cohesive Zone Model (CZM) is selected as an alternative to prevent some of the complications caused by the material heterogeneity and the singularity at the crack-tip. In-spite CZM is a damage based approach, it can be linked to the LEFM which is particularly useful for fatigue modelling. In the second part, the implementation of CZM in XFEM for quasi-static and fatigue modelling is presented. Unlike previous FE implementations of CZM [14, 136], the current approach does not include the undamaged material in the traction separation law to avoid enriching undamaged elements. For the high-cyclic fatigue model, a thermodynamically consistent approach links the Paris law crack growth rate to the damage evolution. A new numerical approach is proposed for the implementation of the CZM for quasi-static and fatigue fracture modelling in XFEM. The outcomes are then compared to the results of other experimental and numerical studies. The fatigue test results comply to the Paris law predictions however, linking Paris law with the damage evolution in the cohesive zone is prone to produce errors since different parts of the cohesive zone undergo different degradation rates

Fatigue crack propagation in functionally graded materials

Propagation of cracks in functionally graded materials (FGMs) under cyclic loading was investigated via experiments and finite element (FE) analysis. Alumina-epoxy composites with an interpenetrating-network structure and tailored spatial variation in composition were produced via a multi-step infiltration technique. Compressed polyurethane foam was infiltrated with alumina slip. After foam burn-out and sintering, epoxy was infiltrated into the porous alumina body. Non-graded specimens with a range of compositions were produced, and elastic properties and fatigue behaviour were characterised. An increase in crack propagation resistance under cyclic loading was quantified via a novel analytical approach. A simulation platform was developed with the commercial FE package ANSYS. Material gradient was applied via nodal temperature definitions. Stress intensity factors were calculated from nodal displacements near the crack-tip. Deflection criteria were compared and the local symmetry criterion provided the most accurate and efficient predictions. An automated mesh-redefinition algorithm enabled incremental simulation of crack propagation. Effects of gradient and crack-geometry parameters on crack-tip stresses were investigated, along with influences of crack-shape, crack-bridging, residual stresses and plasticity. The model provided predictions and data analysis for experimental specimens. Fatigue cracks in graded specimens deflected due to elastic property mismatch, concordant with FE predictions. In other FGMs, thermal or plastic properties may dominate deflection behaviour. Weaker step-interfaces influenced crack paths in some specimens; otherwise effects of toughness variation and gradient steps on crack path were negligible. Crack shape has an influence, but this is secondary to that of elastic gradient. Cracks in FGM specimens initially experienced increase in fatigue resistance with crack-extension followed by sudden decreases at step-interfaces. Bridging had a notable effect on crack propagation resistance but not on crack path. Similarly, crack paths did not differ between monotonic and cyclic loading, although crack-extension effects did. Recommendations for analysis and optimisation strategies for other FGM systems are given. Experimental characterization of FGMs is important, rather than relying on theoretical models. Opportunities for optimization of graded structures are limited by the properties of the constituent materials and resultant general crack deflection behaviour

Industrial Bridge Engineering -Structural developments for more efficient bridge construction

In recent years, growing concern about the deficiencies and lack of efficiency in the construction industry has highlighted the need of research in order to make substantial improvements, rectify incongruities and succeed in progressive development. For bridges, the advantages that can be expected from an industrial construction process are especially interesting. The multidisciplinary research presented in this thesis investigates different means of creating an industrial process for bridge construction while emphasising the vast importance of structural engineering in combination with industrial issues, i.e. industrial bridge engineering. Applications of new or approved techniques, materials technology and developments, methods of design and analysis, as well as construction methods are important areas that have been considered. The increasing utilisation of information and communication technology (ICT), along with more advanced computer-based analysis and simulation methods, are contemporary trends. Seemingly, an important key to the successful construction concepts of tomorrow is to combine these factors into an efficient industrial process. One objective in the presented work has been to establish the foundations for attaining such a new process – basically through analysis of the current bridge construction process – as well as to determine the required improvements needed to provide a framework for a new industrial process. Deficiencies in the traditional bridge construction process have been recognised, as have the underlying driving forces of change. Three cornerstones of industrial bridge construction have been identified – process development, product development and productivity development – and technical necessities have been investigated. Furthermore, a study of an innovative jointing technology for connections between prefabricated concrete elements in bridges has been conducted. The aim has been to design a joint that makes the surrounding elements continuous, but still a very small joint that is easy and fast to perform, and thus highly suitable for use in industrial bridge concepts. In addition, a feasibility study of a novel industrial bridge concept has been undertaken. The concept embraces ultra-high-performance steel-fibre-reinforced concrete in composite action with fibre-reinforced polymers. Several laboratory tests and finite element analyses have been conducted. The main focus has been to investigate new or approved techniques from a design viewpoint as a continuation of recent materials developments, while also considering industrial aspects and construction characteristics such as production methods, assembly, etc

Performance of concrete structures retrofited with CARDIFRC RTM after thermal cycling

A new retrofitting technique using CARDIFRCRTM, a material compatible with concrete, has recently been developed at Cardiff University. It overcomes some of the problems associated with the current techniques based on externally bonded steel plates and FRP (fibre-reinforced polymer), which are due to the mismatch of their tensile strength and stiffness with that of concrete structure being retrofitted. This study investigates the effect of thermal cycles on the performance of reinforced concrete control and retrofitted beams. The concrete beams were heated to a maximum temperature of 90°C from the room temperature of about 25°C. The number of thermal cycles varied from 0 to 90 cycles. After the requisite number of thermal cycles, the beams were tested at room temperature in four-point bending. The tests indicate that the retrofitted beams are stronger, stiffer and more importantly failed in flexure. No visual deterioration or bond degradation was observed after thermal cycling of the retrofitted beams (the bond between the repair material and the concrete substrate remained intact) attesting to the good thermal compatibility between the concrete and CARDIFRCRTM. Therefore, this type of retrofit material can be successfully used in hot climates. The study also evaluates the performance of normal and high strength concretes repaired with CARDIFRCRTM using the wedge splitting test (WST). The main factors that could affect the bond between the repair material and concrete such as the surface roughness and thermal cycling are also investigated. It is shown that surface roughness plays a significant role in the overall bonding system, and no visual deterioration is observed after thermal cycling. Two analytical/computational models for predicting the ultimate moment capacity and the complete load-deflection behaviour of the retrofitted beams were applied. Both models predict very well the ultimate moment capacity of the retrofitted beams